Method of driving a plasma display panel

ABSTRACT

The present invention relates to driving a plasma display panel including discharge cells each corresponding to a pixel in response to a video signal including fields wherein each field is formed by a plurality of subfields, wherein a sustain-level signal (AS) is applied (Q) to cause a sustaining discharge in a discharge cell for emitting light therefrom, and an error diffusion process is carried out. In the error diffusion process (Q −1 , ST, F, A), sustain-level quantization errors of the current filed are detected (Q −1 , ST) and transferred (F, A) to a next field, in which the sustain-level quantization errors are preferably compensated for.

FIELD OF THE INVENTION

The present invention relates to a method and device for driving aplasma display panel including discharge cells each corresponding to apixel in response to a video signal including fields wherein each fieldis formed by a plurality of subfields, comprising applying asustain-level signal to cause a sustaining discharge in a discharge cellfor emitting light therefrom, and carrying out an error diffusionprocess. Further, the present invention relates to a plasma displaypanel apparatus that comprises the mentioned device.

BACKGROUND OF THE INVENTION

In recent years, a thin display apparatus has been requested inconjunction with an increase in size of the display panel. The plasmadisplay panel (hereinafter simply referred to as “PDP”) is expected tobecome one of the most important display devices of the next generationwhich replaces the conventional cathode ray tube, because the PDP caneasily realize reduction of thickness and weight of the panel and theprovision of a flat screen shape and a large screen surface.

In the PDP that makes a surface discharge, a pair of electrodes isformed on an inner surface of a front glass substrate and a rare gas isfilled within the panel. When a voltage is applied across theelectrodes, a surface discharge occurs at the surface of a protectionlayer and a dielectric layer formed on the electrode surface, therebygenerating ultraviolet rays. Fluorescent materials of the three primarycolors red, green and blue are coated on an inner surface of a backglass substrate, and a color display is made by exciting the lightemission from the fluorescent materials responsive to the ultravioletrays.

The PDP comprises a plurality of column electrodes (address electrodes)and a plurality of row electrodes arranged so as to intersect the columnelectrodes. Each of the row electrodes pairs and the column electrodesare covered by a dielectric layer against a discharge space and have astructure such that a discharge cell corresponding to one pixel isformed at an intersecting point of the row electrode pair and the columnelectrode. Since the PDP provides a light emission display by using adischarge phenomenon, each of the discharge cells has only two states; astate where the light emission is performed and a state where it is notperformed. A sub-field method is used to provide a halftone luminancedisplay by the PDP. In the sub-field method, a display period of onefield is divided into N sub-fields, a light emitting period having aduration period corresponding to a weight of each bit digit of the pixeldata (N bits) is allocated every sub-field, and the light emissiondriving is performed.

The discharge is achieved by adjusting voltages between the column androw electrodes of a cell composing a pixel. The amount of dischargedlight changes to adjust the number of discharges in the cell. Theoverall screen is obtained by driving in a matrix type a write pulse forinputting a digital video signal to the column and row electrodes of therespective cells, a scan pulse for scanning a sustain pulse forsustaining discharge, and an erase pulse for terminating discharge of adischarged cell. Also, a gray scale is implemented by differentiatingthe number of discharges of each cell for a predetermined time requiredfor displaying the entire picture.

The luminance of a screen is determined by the brightness for the casewhen each cell is driven to a maximum level. To increase the luminance,a driving circuit must be constructed such that the discharge time of acell can be maintained as long as possible for a predetermined timerequired for forming a screen. The contrast, which is a difference inlight and darkness, is determined by brightness and luminance of abackground such as illumination. To increase the contrast, thebackground must be dark and the luminance thereof must be increased.

In common PDP display systems, a frame or field of a video signalinformation is displayed as a set of subfields. The subfields are oftendriven according the Address Display Separated (ADS) driving scheme.Each subfield has its own address, sustain and erase period. The eraseperiod produces a small quantity of light on the complete display area.Active addressing of a pixel-element creates one light-flash in theaddressed pixel-element. Only the sustain-period generates light onrequest, controlled by a number of sustain-pulses. Each sustain-pulsegenerates two discharges representing a pair of light-flashes.

The ratio of luminance values for each of the subfields depends on theselected subfield distribution in the subfield generation process. Thetotal number of sustain-pulses per frame or field may vary, depending onparameters like power-supply-load, subfield-image load andpanel-temperature. These input parameters are processed, and the totalnumber of sustain-pulses per frame or field is calculated by a microcontroller. In this process the total number of sustain-pulses per frameor field must be converted to a sustain-level per subfield(SF-sustain-level), expressed as a discrete number of sustain pulses.The exact subfield distribution must be maintained during the completeprocess, while the luminance ratio of the subfields must be preserved.Otherwise image artifacts will occur.

However, conventional sub-field distributions used in ADS systems arenot always accurate. They not only suffer from limited gray-levels, butalso have mismatches in their representation.

For panels with limited number of subfields or large amount ofdithering, the SF-sustain-level may have a rather big quantizationerror. When displaying e.g. a gray-scale bar from dark to light, thiscan lead to a non-monotone rising light generation along the gray-scale,causing visible PDP imaging artifacts.

U.S. Pat. No. 6,144,364 A discloses a display driving method whichdrives a display to make a gradation display on a screen of the displaydepending on a length of a light emission time in each of sub fieldsforming 1 field, where 1 field is a time in which an image is displayed,N subfields form 1 field, and each subfield includes an addressdisplay-time in which a wall charge is formed with respect to all pixelswhich are to emit light within the subfield and a sustain time which isequal to the light emission time and determines a luminance level. Thedisplay driving method includes the steps of setting the sustain timesof each of the subfields approximately constant within 1 field, anddisplaying image data on the display using N+1 gradation levels from aluminance level 0 to a luminance level N.

In U.S. Pat. No. 6,175,194 B1 a method for driving a plasma displaypanel is described wherein error diffusion and sustaining pulse controlare used to reduce noise and prevent erroneous discharge to improve thedisplay quality.

In U.S. Pat. No. 5,898,414 A, a display apparatus permitting highresolution and a large number of gray-scale levels and causingindiscernible flicker has been disclosed. One frame is divided into orcomposed of j subframes, and light is produced according to a luminancelevel predetermined subframe by subframe in order to expressintermediate gray-scale of a picture. Emphasis is put on the fact that adisplay to be performed during each subframe within one frame can becontrolled independently. An interlaced-scanning display is carried outduring k subframes associated with low-order weighted bits out of jsubframes, and a non-interlaced-scanning display is carried out duringthe other j-k subframes associated with high-order weighted bits. Theratio of an addressing scan time to a subframe associated with a smallweight is large, and the ratio of an addressing scan time to a wholeframe is very large. If the addressing scan time can be reduced asmentioned above, a great effect would be exerted. Moreover, theluminance levels to be determined in relation to the subframes duringwhich interlaced-scanning display is carried out are so low that theinfluence of the reduction on a whole picture is limited.

U.S. Pat. No. 6,052,101 A describes a driving circuit for plasma displaydevice and a gray scale implementing method therefore. The methodincludes the steps of dividing total horizontal lines of one frame intoX×Y subframes according to a relative luminance ratio, dividing eachframe into X subfields and allotting Y different subframes to eachsubfield, and supplying corresponding gray scale data while sequentiallyerasing each X×Y horizontal lines during one horizontal period from thefirst horizontal electrode lines to the last Nth horizontal electrodelines, included in Y different subframes allotted to each subfield byrepeatedly driving X subfields and scanning the same, therebyimplementing a display picture of 2^(X·Y) gray scales. At least twoscanning and sustaining drivers are provided, and one frame is dividedinto one or more subfields by the drivers, different subframes areallotted to each subfield and then X subfields are repeatedly driven.

SUMMARY OF THE INVENTION

It is an object of the present invention to increase the gray-level orcolor representation for improving the PDP image quality and to providea feasible implementation of such improvement. To this end, theinvention provides a PDP driving as defined by the independent claims.The dependent claims define advantageous embodiments.

The new technique of the present invention can be described asSustain-level Error Diffusion (SED). With this technique thequantization error in the sustain-level generation is omitted, while theremaining error in a subfield sustain-level, hereinafter refer to asSF-sustain-level, is transferred to the next frame and incorporated inthe next SF-sustain-level generation.

It is noted at that in the present text the term “field” can also mean aframe, and the term “subfield” (SF) can also mean a subframe. However,the present invention also covers a situation where a frame of a videosignal consists of subframes, and a subframe consists of subfields.

For a given subfield distribution, only for very specificsustain-levels, all subfields can be displayed with a small quantizationerror. When adaptive regulations are active, sustain-levels can oftennot be accurately mapped due to quantization errors in the individualSF-sustain levels. A smart sustain-level regulation can avoid theseerrors by applying an error diffusion algorithm.

When for each subfield the SF-sustain-level quantization errors areforwarded to the next frame, the total quantization error can beneglected due to the integrating properties of the Human Visual System(HVS).

Preferably, the next frame is a succeeding frame.

The gray-level portrayal of PDP displays can be improved by using theSED technique of the present invention. In case of adaptive luminanceregulation, this technique significantly improves the PDP image quality,while it removes sustain-level to luminance quantization errors. The SEDtechnique of the present invention can be used for all PDP drivingschemes. The implementation of the SED technique of the presentinvention only requires a small software modification of a given PDPdisplay system architecture. So, the present invention provides for afeasible implementation that can be used in combination with other PDPimage improvement algorithms, and, thus, does not add costs.

The sustain-level quantization errors of a specific subfield of acurrent field are transferred to the corresponding subfield of the nextfield. Hence, the technique is independent of any applied subfielddistribution.

In a further preferred embodiment of the present invention, wherein theapplying steps includes the generation of a SF-sustain-level, thetransferred SF-sustain-level quantization errors are incorporated intothe SF-sustain-level generation of the next frame.

In particular, the SF-sustain-level quantization errors are added to therequested SF-sustain-level of the next field.

In a still further preferred embodiment of the present invention therequested SF-sustain-level is generated on the basis of the totalsustain-level signal and SF-distribution. The total sustain-level isdivided over the subfields according to the subfield distribution ratio.It is rounded by a quantization process, and as a result of the roundingstep an actual SF-sustain-level is obtained as an integer number and theremaining part of the requested SF-sustain-level as a quantizationerror. In particular, the requested SF-sustain-level is generated bycalculation, usually by using a micro controller.

Moreover, an adaptive luminance regulation can be used, wherein the SEDtechnique of the present invention significantly improves the PDP imagequality, while it removes sustain-level to luminance quantizationerrors.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the present invention will be described in greaterdetail based on a preferred embodiment with reference to theaccompanying drawings in which

FIG. 1 shows a block diagram of a PDP driving system;

FIG. 2 shows a block diagram of a sustain-level regulation; and

FIG. 3 shows an example of SF-sustain level quantization errors.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An implementation of the Sustain-level Error Diffusion (SED) techniqueis shown as block diagram in FIG. 1. FIG. 1 shows a video processor VP,a sub-field processor SFP, a sub-field load unit SL, a sub-fieldtranspose unit ST, a plasma display panel PDP, a sustain level regulatorSLR, and a timing & control generator T&CG. A temperature T and a powerlimit P are applied to the sustain level regulator SLR.

For each new frame of image-date, the active subfield-pixels are addedto calculate the subfield load. The active subfield load, together withthe power-limits and temperature parameters will determine the totalnumber of sustain-pulses per frame. This is combined with the inputvideo-signal timing and the subfield distribution settings, and a newset of sustain-levels is calculated for each subfield.

While this process is executed by a micro-controller, only softwareneeds to be modified to support the SED technique.

For each frame the new set of sustain levels are forwarded to the Timing& Control process, before the first subfield of the frame is displayed.

When the SF(subfield)-sustain-level is calculated, also thesustain-periods are known. This information is relevant for themotion-compensated subfield calculations. This processes must be awareof the exact timing of each sustain period. It can be considered tomaintain a fixed subfield timing-format and to fill the unusedsustain-period with idle signals.

When for each subfield the SF-sustain-level quantization errors arecompensated for in the next frame, the total quantization error can beneglected.

FIG. 2 schematically shows an embodiment of a sustain-level regulatorSLR where an actual SF-sustain-level is generated on the basis of arequested SF-sustain-level by using a quantization process. In FIG. 2, arequested sustain RS is applied to an adder whose output is applied to aquantizer Q that outputs the actual sustain AS. S is a scaling factor.The actual sustain AS is applied to a de-quantizer Q⁻¹, whose output issubtracted from the input of the quantizer Q by a subtractor ST. Theresulting quantizing error QE is filtered by a filter F, and thereafteradded to the requested sustain by the adder A.

The requested SF-sustain-level SF SL for a subfield is calculated by amicro controller using sustain-level and SF-distribution data, and isexpressed as a number type real. The actual SF-sustain-level SF SL is anumber that must be integer. This implies a quantization process, whichrounds the requested SF-sustain-level SF SL. The remaining part of therequested sustain (type real) is propagated to the related subfield inthe next frame and added to the requested SF-sustain-level of thatframe.

The filter characteristics are only a delay. The delay is a completeframe period minus the active sub-field period.

By providing a Sustain Level Regulation operation and a Timing andControl Generator, the SED technique is applied to forwardSF-sustain-level SF SL errors to the next image field or frame. Thesestages calculate the sustain-levels and sustain-time for each subfieldto adaptively regulate SF-sustain-levels SF SL for the PDP.

FIG. 3 shows an example of a SF distribution with various sustain-levelsSL, namely with a sustain-level SL of 100% without any quantizationerrors QE, and with sustain-levels SL of 140% and 40% with quantizationerrors QE.

Although the invention is described above with reference to an exampleshown in the attached drawings, it is apparent that the invention is notrestricted to it, but can vary in many ways within the scope disclosedin the attached claims. In the claims, any reference signs placedbetween parentheses shall not be construed as limiting the claim. Theword “comprising” does not exclude the presence of elements or stepsother than those listed in a claim. The word “a” or “an” preceding anelement does not exclude the presence of a plurality of such elements.In the device claim enumerating several means, several of these meanscan be embodied by one and the same item of hardware. The mere fact thatcertain measures are recited in mutually different dependent claims doesnot indicate that a combination of these measures cannot be used toadvantage.

1. A method for driving a plasma display panel including discharge cellseach corresponding to a pixel in response to a video signal includingfields wherein each field is formed by a plurality of subfields, themethod comprising: applying a sustain-level signal to cause a sustainingdischarge in a discharge cell for emitting light therefrom, and carryingout an error diffusion process, characterized in that the errordiffusion process comprises detecting sustain-level quantization errors,and transferring the sustain-level quantization errors of the currentfield to a next field.
 2. A device for driving a plasma display panelincluding discharge cells each corresponding to a pixel in response to avideo signal including fields wherein each field is formed by aplurality of subfields, the device comprising: means for applying asustain-level signal to cause a sustaining discharge in a discharge cellfor emitting light therefrom, and means for carrying out an errordiffusion process, characterized in that the error diffusion processcarrying out means comprise means for detecting sustain-levelquantization errors, and means for transferring the sustain-levelquantization errors of a current field to a next field.
 3. The deviceaccording to claim 2, characterized in that the sustain-levelquantization errors are compensated for in the next field.
 4. The deviceaccording to claim 2, characterized in that the transferring meanstransfer the sustain-level quantization errors of a predeterminedsubfield of the current field to the corresponding subfield in the nextfield.
 5. The device according to claim 2, wherein the applying meansgenerate a sustain-level, and the transferring means incorporate thetransferred sustain-level quantization errors into the next fieldsustain-level generation.
 6. The device according to claim 4,characterized in that the transferring means incorporate the transferredsustain-level quantization errors into the next subfield sustain-levelgeneration.
 7. The device according to claim 2, characterized in thatthe transferring means add the transferred sustain-level quantizationerrors to the requested sustain-level of the next field.
 8. The deviceaccording to claim 2, further comprising means for generating arequested sustain-level on the basis of sustain-level and subfielddistribution data, quantization process means for rounding the requestedsustain-level by a quantization process, and means for generating anactual sustain-level as an integer number and the remaining part of therequested sustain-level as a quantization error in accordance with theresult of the quantization process.
 9. The device according to claim 2,further comprising luminance regulation means, preferably an adaptiveluminance regulation means.
 10. A plasma display panel apparatuscomprising the device according to claim 2.